An engine (of a vehicle), such as an internal-combustion engine (ICE), has a heat generating assembly, such as a combustion chamber. The combustion chamber facilitates combustion of a fuel (such as a fossil fuel) with an oxidizer (such as air). The combustion chamber may be recessed in a cylinder head of the engine and contains an intake valve and an exhaust valve. Some engines use a dished piston and in this case, the combustion chamber is a part of a cylinder that slidably receives the dished piston. After fuel ignition, the combusting fuel and oxidizer mixture acts upon the piston in such a way as to push the piston in a direction of the expending combusting gas (fuel).
In the internal-combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion (in the combustion chamber) apply a direct force to a movable component (such as a piston assembly) of the engine. This force moves the component over a distance, transforming chemical energy into useful mechanical energy. The term internal-combustion engine usually refers to an engine in which combustion is intermittent, such as the four-stroke piston engine and/or the two-stroke piston engine, along with variants, such as the six-stroke piston engine and the Wankel rotary engine and equivalents thereof.
Another class of internal-combustion engines use continuous combustion: gas turbines, jet engines, and rocket engines, each of which are internal-combustion engines that are configured to operate under the same principle as previously described. The internal-combustion engine is different from known external-combustion engines, such as the steam engine or Stirling engines, in which the energy is delivered to a working fluid (cooling medium) not consisting of, mixed with, or contaminated by combustion products. Working fluids can be air or some noble gases, hot water, pressurized water or even liquid sodium, heated in some kind of boiler. Internal-combustion engines are usually powered by energy-dense fuels such as gasoline or diesel, or liquids derived from fossil fuels. While there are many stationary applications, most internal-combustion engines are used in mobile applications and are the dominant power supply for cars, aircraft, and boats.
Two common forms of engine cooling are air-cooled and water-cooled. Most modern engines are water-cooled. Some engines (air cooled or water cooled) also have an oil cooler. Cooling is required to remove excessive heat from the engine. Over-heating of the engine may cause engine failure, usually from wear, cracking or warping. The term “internal-combustion engine cooling” refers to the cooling of the internal-combustion engine, typically using either air or a liquid. Typically, internal-combustion engines of a car may use water for cooling (if so desired).
Heat engines (also known as the engine or the internal-combustion engine, etc.) generate mechanical power by extracting energy from expanding gas generated by internal combustion, much as a water wheel extracts mechanical power from a flow of mass falling through a distance. Because of the enclosed combustion process, an engine of a car (vehicle) operates inefficiently, so considerably more fuel chemical energy enters the engine than comes out as mechanical power; the difference is waste heat that must be removed. The internal-combustion engine is configured to remove waste heat through heat absorption by cool intake air, quick removal of hot exhaust gases, explicit engine cooling and by simply radiating energy from a highly conductive engine block and associated connections. Lubricating oil removes a relatively small portion of engine heat as well. Engines with a higher efficiency have more energy that leaves as mechanical motion and less as waste heat.
Some waste heat may be removed from the cabin of the automobile (vehicle), so that the driver (vehicle operator) is comfortable during prolonged driving in a relatively higher environmental temperature. This heat is considered lost since a compressor driven by an engine shaft directly removes this energy. Passenger-cabin cooling is a feature of a vehicle, and may be supplied as a standard option.
Heat engines need cooling to operate properly. Cooling is also needed because high temperatures may lead to inadvertent damage to engine materials and lubricants. Internal-combustion engines burn fuel hotter than the melting temperature of engine materials, and hot enough to set fire to the lubricants. Engine cooling removes energy fast enough to keep temperatures low so the engine can survive and operate reliably. Good control over the operating temperature of the engine is an important aspect for engine performance and efficiency.
Some high-efficiency engines operate without explicit cooling, and with only accidental heat loss in accordance with a design called adiabatic. For example, 10,000 mile-per-gallon cars are insulated; both to transfer as much energy as possible from hot gases to mechanical motion, and to reduce reheat losses when restarting. Such engines can achieve high efficiency by impacting power output, duty cycle, engine weight, durability, and/or emissions.
Most internal-combustion engines are fluid cooled using either air (a gaseous fluid) or a liquid coolant that runs through a heat exchanger (radiator) cooled by air. Marine engines and some stationary engines have ready access to a large volume of water at a suitable temperature. The water may be used directly to cool the engine, but often has sediment that may inadvertently clog coolant passages, or chemicals, such as salt, minerals and deposits that can chemically damage the engine. Thus, engine coolant may be run through a heat exchanger that is cooled by the body of water in order to avoid inadvertent damage in the engine.
Most liquid-cooled engines use a mixture of water and chemicals such as antifreeze and rust inhibitors. The term for the antifreeze mixture is engine coolant. Some antifreezes use no water at all, instead using a liquid with different properties, such as propylene glycol or a combination of propylene glycol and ethylene glycol. Most air-cooled engines use some liquid oil cooling, to maintain acceptable temperatures for both critical engine parts and the oil itself. Most liquid-cooled engines use some air cooling, with the intake stroke of air used for combustion. The heat energy absorbed by cold intake air is lost energy, and is not recovered due to heating of the intake air. Gaseous cooling for the engine is not capable, by sensible heat only, to remove all the heat generated by the internal-combustion process. Water has high-heat capacity and is a good coolant medium. Water requires large-size conductive channels so that the water may flow freely within the engine block. The water cooling operates at the very critical temperature, close to 100 degrees Centigrade when water boils. Boiling water is undesirable for engine cooling. This very nature of current ICE cooling, operating around water critical point, may be limiting.
There are many demands on a cooling system of the engine. One requirement is that an engine may fail if just one part of the engine overheats. Therefore, it is vital that the cooling system of the engine keeps all parts of the engine at suitably stable temperatures and at an efficient operating point. Liquid-cooled engines are able to vary the size of their passageways through the engine block so that coolant flow may be tailored for the needs of each area. Locations with either high peak temperatures (narrow islands around the combustion chamber) or high-heat flow (around exhaust ports) may require generous cooling. This reduces the occurrence of hot spots, which are more difficult to avoid with air cooling. Air-cooled engines may also vary their cooling capacity by using more closely spaced cooling fins in that area, but this can make their manufacture difficult and expensive. Besides, cooling air temperature may very significantly during engine operation.
Some parts of the engine, such as the engine block and head, are cooled directly by the main coolant system. Moving parts such as the pistons, and to a lesser extent the crank and rods, must rely on the lubrication oil as a coolant, or to a very limited amount of conduction into the engine block and thence the main coolant. High-performance engines frequently have additional oil, beyond the amount needed for lubrication, sprayed upwards onto the bottom of the piston just for extra cooling. This oil is then air cooled via air heat exchanger. Therefore, the heat energy is expelled into an environment and is not recovered.
Liquid-cooled engines usually have a circulation pump. The first engines relied on thermo-syphon cooling alone, where hot coolant left a top of the engine block and passed to the radiator, where it was cooled before returning to the bottom of the engine. Circulation was powered by convection alone.
Other demands include other factors such as cost, weight, reliability, and durability of the cooling system itself. Cooling with water requires large liquid channels, and that makes engine coolant-fluid containment relatively bigger and heavier. This adds weight to moving vehicles and adds to overall burden to engine efficiency, and lowers fuel efficiency of the car.
Conductive heat transfer is proportional to the temperature difference between materials. If the engine metal is at 250° C. (degrees Centigrade) and the air is at 20° C., then there is a 230° C. temperature difference for cooling. An air-cooled engine uses all of this difference. In contrast, a liquid-cooled engine might dump heat from the engine to a liquid, heating the liquid to 135° C. (the standard boiling point of water is 100° C. and can be exceeded as the water cooling system is allowed to be both pressurized, and uses a mixture with antifreeze) which is then cooled with 20° C. air. In each step, the liquid-cooled engine has half the temperature difference and so at first appears to need twice the cooling area.
However, properties of the coolant (water, oil, or air) also affect cooling. For example, comparing water and oil as coolants, one gram of oil can absorb about 55% of the heat for the same rise in temperature (called the specific heat capacity). Oil has about 90% the density of water, so a given volume of oil can absorb only about 50% of the energy of the same volume of water. The thermal conductivity of water is about four times that of oil, which can assist in heat transfer. The viscosity of oil can be ten times greater than water, increasing the energy required to pump oil for cooling, and reducing the net power output from the engine.
Comparing air and water, air has a vastly lower heat capacity per gram and per volume, and less than a tenth the conductivity, but also much lower viscosity (about 200 times lower). Therefore, air-cooling needs ten times the surface area, therefore, the fins, and the air needs about 2000 times the flow velocity and thus the recirculating air fan may need ten times the power of a recirculating water pump. It may be desirable to eliminate cooling fans and coolant pumps (to improve reliability of the engine).
Moving heat from the cylinder to a large surface area for air cooling can present problems such as difficulties associated with manufacturing the shapes needed for good heat transfer and the space needed for free flow of a large volume of air. Water boils at about the same temperature desired for engine cooling. This has the advantage that it absorbs a great deal of energy with a relatively little rise in temperature (called the heat of vaporization), which is good for keeping things cool; however, this is not utilized for cooling internal-combustion engines due to size and weight requirements. In moving vehicles, this may also be very inefficient.
In contrast, passing air over several hot objects in series warms the air at each step, so the first step may be over-cooled and the last step may be under-cooled. However, once water boils, if vaporized water is not removed and cooled, it acts an insulator, leading to a sudden loss of cooling where steam bubbles form; unfortunately, steam may return to water as it mixes with other coolants, so an engine temperature gauge can indicate an acceptable temperature even though local temperatures are high enough that damage is done to the engine.
The parts of the engine need different temperatures. For example, the inlet includes a compressor of a turbo, inlet trumpets, inlet valves that need to be as cold as possible for proper operation. A countercurrent heat exchange with forced cooling air may assist in this requirement. The cylinder-walls should not heat up the air before compression, but also not cool down the gas in the combustion chamber. Operating temperature of the internal-combustion engine is set due to limits of cooling water and not due to efficiency of energy conversion. Since water is used for cooling with boiling temperature at 100° C., a compromise is established so that a cylinder wall temperature is around 90° C. Then, the viscosity of the oil is optimized for just this temperature. Any cooling of the exhaust and the turbine of the turbocharger reduces the amount of power available to the turbine, so the exhaust system is often insulated between engine and turbocharger to keep the exhaust gases as hot as possible.
The temperature of the cooling air may range from well below freezing to 50° C. Further, while engines in long-haul boat or rail service may operate at a steady load, road vehicles often see widely varying and quickly varying load. Thus, the cooling system is designed to vary cooling so the engine is neither too hot nor too cold. Cooling-system regulation includes adjustable baffles in the air flow (sometimes called shutters and commonly run by a pneumatic shutter). A fan operates either independently of the engine, such as an electric fan, or which has an adjustable clutch. A thermostatic valve (also called a thermostat) can block the coolant flow when conditions are too cool. In addition, the motor, coolant, and heat exchanger have some heat capacity, which smoothens out temperature increase in short sprints. Some engine controls shut down an engine or limit engine operation to half throttle if the engine overheats. Some electronic engine controls adjust cooling based on a throttle condition to anticipate a temperature rise, and limit engine power output to compensate for finite cooling. Accurate engine temperature control is relatively nonexistent.
It is usually desirable to minimize the number of heat transfer stages in order to maximize the temperature difference at each stage. However, some diesel two-stroke cycle engines use oil cooled by water, with the water in turn cooled by air. The coolant used in many liquid-cooled engines must be renewed periodically, and can freeze at ordinary temperatures thus causing permanent engine damage.
Cars and trucks using direct air cooling (without an intermediate liquid) were built over a long period from the very beginning, and ending with a small and generally unrecognized technical change. Before World War II, water-cooled cars and trucks routinely overheated while climbing mountain roads, creating geysers of boiling water. This was considered normal, and at the time, most noted mountain roads had auto repair shops to minister to overheated engines.
During that period, some car manufacturers built diesel trucks, farm tractors, and passenger cars that were air-cooled. Air-cooled engines may be adapted to extremely cold and hot environmental weather temperatures. Air-cooled engines may start and run in freezing conditions (in which water-cooled engines cannot since they may become stuck), and continue working when water-cooled engines start producing unwanted leakage in the form of steam jets. Furthermore, with the possibility of working at higher temperatures, air-cooled engines may have an advantage from a thermodynamic point of view. A problem met in air-cooled aircraft engines was the so-called shock cooling when an airplane entered in a dive after climbing or leveled flight with the throttle opened. With the engine under no-load while the airplane dives, the engine generates less heat, and the flow of air that cools the engine is increased. A catastrophic engine failure may result as different parts from the engine have different temperatures, and thus different thermal expansions. In such conditions, the engine may get stuck or seize, and any sudden change or imbalance in the relation between heat produced by the engine and heat dissipated by cooling may result in an increased wear in the engine, as a consequence also of thermal dilatation differences between parts from the engine may cause the engine to inadvertently crack.
Liquid cooled engines have more stable and uniform working temperatures, and are less susceptible to variation in air temperatures. Most engines are liquid-cooled. Liquid cooling is also employed in maritime vehicles (vessels). For vessels, the seawater, itself is mostly used for cooling. In some cases, chemical coolants are also employed (in closed systems), or they are mixed with seawater cooling. While liquid cooling in general has some advantages, it may require larger cooling passages and tends to operate at the smaller temperature differential. As well, the optimal operating temperature of the engine may be outside the water cooling operating range.
The change from air cooling to liquid cooling occurred at the start of World War II when the military needed more reliable vehicles. The subject of boiling engines was addressed, researched, and a solution was found. Previous radiators and engine blocks were properly designed and survived durability tests, but used water pumps with a leaky graphite-lubricated rope seal (gland) on a pump shaft. The seal was inherited from steam engines, where water loss is accepted since steam engines already expend large volumes of water. Because the pump seal leaked mainly when the pump was running and the engine was hot, the water loss evaporated inconspicuously, leaving at best small rusty traces when the engine stopped and cooled, thereby not revealing significant water loss. Automobile radiators (or heat exchangers) have an outlet that feeds cooled water to the engine, and the engine has an outlet that feeds heated water to the top of the radiator. Water circulation is aided by a rotary pump that has only a slight effect, having to work over such a wide range of speeds that its impeller has only a minimal effect as a pump. While running, the leaking pump seal drained cooling water at a level where the pump could no longer return water to the top of the radiator, so water circulation ceased and water in the engine boiled. However, since water loss led to engine overheating and further water loss from boil-over, the original water loss was hidden.
After isolating the pump problem, cars and trucks built for the war effort were equipped with carbon-seal water pumps that did not leak and caused fewer inadvertent geysers. Meanwhile, air cooling advanced in memory of boiling engines even though boil-over was no longer a common problem. Air-cooled engines became popular throughout Europe. As air quality awareness rose in the 1960s, and laws governing exhaust emissions were passed, unleaded gas replaced leaded gas, and leaner fuel mixtures became the norm. These reductions in the cooling effects of both the lead and the formerly rich fuel mixture, led to overheating of the air-cooled engines. Valve failures and other engine damage resulted. One manufacturer responded by abandoning their (flat) horizontally opposed air-cooled engines, while another manufacturer chose liquid cooling for their engine when it was introduced.
However, many motorcycles use air cooling for the sake of reducing weight and complexity. Some automobiles have air-cooled engines, but historically, it was common for many high-volume vehicles to be preferably cooled by air.
Most aviation piston engines are air-cooled, including most of the engines currently manufactured and used by major manufacturers of aircraft but there are some exceptions.
Other engine manufacturers use a combination of air-cooled cylinders and liquid-cooled cylinder heads.